hi Kerrybelle,
I don't really have any advice about what kind of bike, but I think you'd be just as safe on a used bike that has had a good check-up from your lbs as with a new one that would need cables tightened after a short while anyway. If you're unhappy about traffic I'd say pick a comfy robust bike with decent components, not too heavy, that lets you see what's going on around you well, and spend a little money on lights, fenders, maybe a mirror and a rear rack too. I prefer commuting on my mtb because I can see more and react quicker with it and it can handle potholes and sidewalk hopping well.
Commuting by bike rocks, have fun
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett